Module 6 - Industrial Instrumentation and Automation:

H330 Automotive Engineering MODULE 5 OVERVIEW OF AUTOMATION SYSTEM - ARCHITECTURE OF INDUSTRIAL AUTOMATION SYSTEMS, DIFFERENT DEVICES USED IN AUTOMATION, ACTUATORS - DEFINITION, TYPES, SELECTION. PNEUMATIC, HYDRAULIC, ELECTRICAL AND ELECTRO-PNEUMATIC VALVES , SHAPE MEMORY ALLOYS Automation - Automation is the technology by which a process or procedure is performed with minimal human assistance - An automatic control uses various control systems for operating equipment, machinery, processes in factories, boilers and heat treating ovens, switching on telephone networks, steering and stabilization of ships, aircraft and other applications and vehicles with minimal or reduced human intervention - It is implemented using a program of instructions combined with a control system that executes the instructions, to automate a process. - Power is required, both to drive the process itself and to operate the program and control system. Advantages of automation • Increased productivity. • Improved quality or increased predictability of quality. • Improved robustness (consistency), of processes or product. • Increased consistency of output. • Reduced direct human labor costs and expenses. 2 • Can complete tasks where a high degree of accuracy is required. • Replaces human operators in tasks that involve hard physical or monotonous work (e.g., using one forklift with a single driver instead of a team of multiple workers to lift a heavy object) • Reduces some occupational injuries (e.g., fewer strained backs from lifting heavy objects) • Replaces humans in tasks done in dangerous environments (i.e. fire, space, volcanoes, nuclear facilities, underwater, etc.) • Performs tasks that are beyond human capabilities of size, weight, speed, endurance, etc. • Reduces operation time and work handling time significantly. • Frees up workers to take on other roles. • Provides higher level jobs in the development, deployment, maintenance and running of the automated processes. Disadvantages Possible security threats/vulnerability due to increased relative susceptibility for committing errors. Unpredictable or excessive development costs. High initial cost. Displaces workers due to job replacement. 3 The Functional Elements of Industrial Automation - An Industrial Automation System consists of numerous elements that perform a variety of functions related to Instrumentation, Control, Supervision and Operations Management related to the industrial process. - These elements may also communicate with one another to exchange information necessary for overall coordination and optimized operation of the plant/factory/process. - Below, we classify the major functional elements typically found in IA systems and also describe the nature of technologies that are employed to realize the functions. 1. Sensing and Actuation Elements - These elements interface directly and physically to the process equipment and machines. - The sensing elements translate the physical process signals such as temperature, pressure or displacement to convenient electrical or pneumatic forms of information, so that these signals can be used for analysis, decisions and finally, computation of control inputs. - These computed control inputs, which again are in convenient electrical or pneumatic forms of information, need to be converted to physical process inputs such as, heat, force or flow-rate, before they can be applied to effect the desired changes in the process outputs. - Such physical control inputs are provided by the actuation elements. 4 1.1 Industrial Sensors and Instrument Systems - Scientific and engineering sensors and instrument systems of a spectacular variety of size, weight, cost, complexity and technology are used in the modern industry. - However, a close look would reveal that all of them are composed of a set of typical functional elements connected in a specified way to provide signal in a form necessary for the various tasks involved in the automation systems. - Figure below shows the configuration of a typical sensor system. - In Figure, a sensor system is shown and is decomposed into four of its major functional components, along with the medium in which the measurement takes place. These are described below. 5 a) The physical medium - It refers to the object where a physical phenomenon is taking place and we are interested in the measurement of some physical variable associated with the phenomenon. - Thus, for example, the physical medium may stand for the hot gases in a furnace in the case of temperature measurement or the fluid in a pipe section in the case of measurement of liquid flow rate. b) The sensing element - It is affected by the phenomenon in the physical medium either through direct or physical contact or through indirect interaction of the phenomenon in the medium with some component of the sensing element. - Again, considering the case of temperature measurement, one may use a thermocouple probe as the sensing element that often comes in physical contact with the hot object such as the flue gas out of a boiler-furnace or an optical pyrometer which compares the brightness of a hot body in the furnace with that of a lamp from a distance through some window and does not come in direct contact with the furnace. - In the more common case where the sensing element comes in contact with the medium, often some physical or chemical property of the sensor changes in response to the measurement variable. - This change then becomes a measure of the physical variable of interest. 6 - A typical example is the change in resistivity due to heat in a resistance thermometer wire. - Alternatively, in some other sensors a signal is directly generated in the sensing element, as is the case of a thermocouple that generates a voltage in response to a difference in temperature between its two ends. c) The signal-conditioning element - It serves the function of altering the nature of the signal generated by the sensing element. - Since the method of converting the nature of the signal generated in the sensor to another suitable signal form (usually electrical) depends essentially on the sensor, individual signal conditioning modules are characteristic of a group of sensing elements. - As an example consider a Resistance Temperature Detector (RTD) whose output response is a change in its resistance due to change in temperature of its environment. - This change in resistance can easily be converted to a voltage signal by incorporating the RTD in one arm of a Wheatstone's bridge. - The bridge therefore serves as a signal-conditioning module. - Signal conditioning modules are also used for special purpose functions relating to specific sensors but not related to variable conversion such as `ambient referencing' of 7 thermocouples. - These typically involve analog electronic circuits that finally produce electrical signals in the form of voltage or current in specific ranges. d) The signal processing element - It is used to process the signal generated by the first stage for a variety of purposes such as, filtering (to remove noise), diagnostics (to assess the health of the sensor) and linearization (to obtain an output which is linearly related with the physical measurand etc. - Signal processing systems are therefore usually more general purpose in nature. e) The target signal-handling element - It may perform a variety of functions depending on the target application. - It may therefore contain data/signal display modules, recording or/storage modules, or simply a feedback to a process control system. - Examples include a temperature chart recorder, an instrumentation tape recorder, a digital display or an Analog to Digital Converter (ADC) followed by an interface to a process control computer. 8 1.2 Industrial Actuator Systems - Actuation systems convert the input signals computed by the control systems into forms that can be applied to the actual process and would produce the desired variations in the process physical variables. - In the same way as in sensors but in a reverse sense, these systems convert the controller output, which is essentially information without the power, and in the form of electrical voltages (or at times pneumatic pressure) in two ways. - Firstly it converts the form of the variable into the appropriate physical variable, such as torque, heat or flow. - Secondly it amplifies the energy level of the signal manifold to be able to causes changes in the process variables. - Thus, while both sensors and actuators cause variable conversions, actuators are high power devices while sensors are not. - It turns out that in most cases, actuators are devices that first produce motion from electrical signal, which is then further converted to other forms. - Based on the above requirement of energy and variable conversion most actuation systems are structured as shown in the figure in next slide 9 - In figure an actuator system is shown decomposed into its major functional components. - The salient points about the structure are described below. a) The electronic signal-processing element - This element accepts the command from the control system in electrical form. - The command is processed in various ways. - For example it may be filtered to avoid applying input signals of certain frequencies that may cause resonance. - Many actuators are themselves closed feedback controlled units for precision of the actuation operation. - Therefore the electronic signal-processing unit often contains the control system for the actuator itself. 10 b) The electronic power amplification element - This element sometimes contains linear power amplification stages called servo-amplifiers. - In other cases, it may comprise power electronic drive circuits such as for motor driven actuators. c) The variable conversion element - It serves the function of altering the nature of the signal generated by the electronic power amplification element from electrical to non-electrical form, generally in the form of motion. - Examples include electrohydraulic servo valve, stepper/servo motors, Current to Pneumatic Pressure converters etc. d) The non-electrical power conversion elements - These are used to amplify power further, if necessary, typically using hydraulic or pneumatic mechanisms. f) Other Miscellaneous Elements - These include elements such as Auxiliaries for Lubrication/Cooling/Filtering, Reservoirs, Prime Movers etc., sensors for feedback, components for display, remote operations, as well as safety mechanisms since the power handling level is significantly high. 11 2. Industrial Control Systems - By industrial control systems, we denote the sensors systems, actuator systems as a controller. - Controllers are essentially (predominantly electronic, at times pneumatic/hydraulic) elements that accept command signals from human operators or Supervisory Systems, as well as feedback from the process sensors and produce or compute signals that are fed to the actuators. - Control Systems can be classified into two kinds. 2.1 Continuous Control - This is also often termed as Automatic Control, Process Control, Feedback Control etc. - Here the controller objective is to provide such inputs to the plant such that the output y(t) follows the input r(t) as closely as possible, in value and over time. - The structure of the common control loop with its constituent elements, namely the Controller, the Actuator, the Sensor and the Process itself is shown in next slide. - In addition the signals that exist at various points of the system are also marked. - These include the command (alternatively termed the set point or the reference signal), the exogenous inputs (disturbances, noise). - The difficulties in achieving the performance objective is mainly due to the unavoidable disturbances due to load variation and other external factors, as well as sensor noise, the complexity, possible instability, uncertainty and variability in the plant dynamics, as well as limitations in actuator capabilities. 13 - Most industrial control loop command signals are piecewise constant signals that indicate desirable levels of process variables, such as temperature, pressure, flow, level etc., which ensure the quality of the product in Continuous Processes. - In some cases, such as in case of motion control for machining, the command signal may be continuously varying according to the dimensions of the product. - Therefore, here deviation of the output from the command signal results in degradation of product quality. - It is for this reason that the choice of the feedback signals, that of the controller algorithm (such as, P, PI pr PID), the choice of the control loop structure (normal feedback loop, cascade loop or feedforward) as well as choice of the controller gains is extremely important for industrial machines and processes. - Typically the control configurations are well known for a given class of process, however, the choice of controller gains have to be made from time to time, since the plant operating characteristics changes with time. - This is generally called controller tuning. 14 2.2 Sequence / Logic Control - Many control applications do not involve analog process variables, that is, the ones which can assume a continuous range of values, but instead variables that are set valued, that is they only assume values belonging to a finite set. - The simplest examples of such variables are binary variables, that can have either of two possible values, (such as 1 or 0, on or off, open or closed etc.). - These control systems operate by turning on and off switches, motors, valves, and other devices in response to operating conditions and as a function of time. - Such systems are referred to as sequence/logic control systems - A modern controller device used extensively for sequence control today in transfer lines, robotics, process control, and many other automated systems is the Programmable Logic Controller (PLC). - In essence, a PLC is a special purpose industrial microprocessor based real-time computing system, which performs the following functions in the context of industrial operations 15 3. Supervisory Control - Supervisory control performs at a hierarchically higher level over the automatic controllers, which controls smaller subsystems. - Supervisory control systems perform, typically the following functions: Set point computation: Set points for important process variables are computed depending on factors such as nature of the product, production volume, mode of processing. This function has a lot of impact on production volume, energy and quality and efficiency. Performance Monitoring / Diagnostics: Process variables are monitored to check for possible system component failure, control loop detuning, actuator saturation, process parameter change etc. The results are displayed and possibly archived for subsequent analysis. Start up / Shut down / Emergency Operations: Special discrete and continuous control modes are initiated to carry out the intended operation, either in response to operator commands or in response to diagnostic events such as detected failure modes. Control Reconfiguration / Tuning: Structural or Parametric redesign of control loops are carried out, either in response to operator commands or in response to diagnostic events such as detected failure modes. Control reconfigurations may also be necessary to accommodate variation of feedback or energy input e.g. gas fired to oil fired. Operator Interface: Graphical interfaces for supervisory operators are provided, for manual 16 supervision and intervention. - Naturally, these systems are dependent on specific application processes, in contrast with automatic control algorithms, which are usually generic (e.g. PID). - Computationally these are a mixture of hard and soft real time algorithms. - These are also often very expensive and based on proprietary knowledge of automating specific classes of industrial plants. 4. Production Control - Production control performs at a hierarchically higher level over the supervisory controllers. - Typical functions they perform are: Process Scheduling: Depending on the sequence of operations to be carried on the existing batches of products, processing resource availability for optimal resource utilization. Maintenance Management: Decision processes related to detection and deployment of maintenance operations. Inventory Management: Decision processes related to monitoring of inventory status of raw material, finished goods etc. and deployment of operations related to their management. Quality Management: Assessment, Documentation and Management of Quality - Typically, the algorithms make use of Resource Optimization Technology and are non-real-time although they may be using production data on-line. 17 The Architecture of Elements: The Automation Pyramid - Industrial automation systems are very complex having large number of devices with confluence of technologies working in synchronization. - In order to know the performance of the system we need to understand the various parts of the system. - Industrial automation systems are organized hierarchically as shown in the following figure. 18 - Various components in an industrial automation system can be explained using the automation pyramid as shown in figure. - Here, various layers represent the wideness (in the sense of no. of devices), and fastness of components on the time-scale. Sensors and Actuators Layer: This layer is closest to the processes and machines, used to translate signals so that signals can be derived from processes for analysis and decisions and hence control signals can be applied to the processes. This forms the base layer of the pyramid also called ‘level 0’ layer. Automatic Control Layer: This layer consists of automatic control and monitoring systems, which drive the actuators using the process information given by sensors. This is called as ‘level 1’ layer. Supervisory Control Layer: This layer drives the automatic control system by setting target/goal to the controller. Supervisory Control looks after the equipment, which may consist of several control loops. This is called as ‘level 2’ layer. Production Control Layer: This solves the decision problems like production targets, resource allocation, task allocation to machines, maintenance management etc. This is called ‘level 3’ layer. Enterprise control layer: This deals less technical and more commercial activities like supply, demand, cash flow, product marketing etc. This is called as the ‘level 4’ layer. 19 - The spatial scale increases as the level is increased e.g. at lowest level a sensor works in a single loop, but there exists many sensors in an automation system which will be visible as the level is increased. - The lowest level is faster in the time scale and the higher levels are slower. - The aggregation of information over some time interval is taken at higher levels. - All the above layers are connected by various types of communication systems. - For example the sensors and actuators may be connected to the automatic controllers using a point-to-point digital communication, while the automatic controllers themselves may be connected with the supervisory and production control systems using computer networks. - Some of these networks may be proprietary. Over the last decade, with emergence of embedded electronics and computing, standards for low level network standards (CANBus, Fieldbus etc.) for communication with low level devices, such as sensors and actuators are also emerging. Device level: This is the lowest level in our automation hierarchy. It includes the actuators, sensors, and other hardware components that comprise the machine level. Machine level: Hardware at the device level is assembled into individual machines. Examples include CNC machine tools and similar production equipment, industrial robots, powered conveyors, and automated guided vehicles. 20 Cell or system level: This is the manufacturing cell or system level, which operates under instructions from the plant level. A manufacturing cell or system is a group of machines or workstations connected and supported by a material handling system, computer and other equipment appropriate to the manufacturing process. Production lines are included in this level Plant level: This is the factory or production systems level. It receives instructions from the corporate information system and translates them into operational plans for production. Enterprise level: This is the highest level consisting of the corporate information system Industrial Example - The die stamping process is shown in figure in next slide. - This process consists of a metal stamping die fixed to the end of a piston. - The piston is extended to stamp a work piece and retracted to allow the work piece to be removed. - The process has 2 actuators: an up solenoid and a down solenoid, which respectively control the hydraulics for the extension and retraction of the stamping piston and die. - The process also has 2 sensors: an upper limit switch that indicates when the piston is fully retracted and a lower limit switch that indicates when the piston is fully extended. - Lastly, the process has a master switch which is used to start the process and to shut it 21 down. 22 Actuator - An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. - In simple terms, it is a "mover". - An actuator requires a control signal and a source of energy. - The control signal is relatively low energy and may be electric voltage or current, pneumatic or hydraulic pressure or even human power. - Its main energy source may be an electric current, hydraulic fluid pressure or pneumatic pressure. - When it receives a control signal, an actuator responds by converting the signal's energy into mechanical motion. - An actuator is the mechanism by which a control system acts upon an environment. - The control system can be simple (a fixed mechanical or electronic system), software-based (e.g. a printer driver, robot control system), a human, or any other input. 23 Selection of actuators - The decision to automate a valve is usually based on one or all of the following considerations. 1. Compatibility (Power Source) - First and foremost in the selection of an actuator type (pneumatic or electric) is to determine the most effective power source for the actuator. - Points to consider are: Power source availability Torque at the valve stem Failure mode Control accessories Speed of operation Frequency of operation Plant environment Size of valve System component costs 24 System maintenance - The most practical pneumatic actuators utilize an air pressure supply of 40 to 120 psi (3 to 8 bar). - Generally they are sized for a supply pressure of 60 to 80 psi (4 to 6 bar). - Higher air pressure is usually difficult to guarantee and lower pressures require a very large diameter piston or diaphragm to generate desirable operating torque. - Electric actuators are often used with a 12 – 24V DC, 24,120,220V ac power supply. 2. Temperature range - Both pneumatic and electric actuators may be used in a wide temperature range. - The standard temperature range of a pneumatic actuator is from -4 to 174°F (-20 to 80°C) but may be extended to -40 to 250°F (-40 to 121°C) with optional seals, bearings and grease. - If control accessories are used (limit switches, solenoid valves etc.) they may not have the same temperature rating as the actuator and this should be considered in all applications. - Electric actuators are available in a temperature range of -40 to 150°F (-40 to 65°C). - When used outdoors an electric actuator should be sealed from the environment to prevent the introduction of moisture to the internal workings. 25 3. Hazardous Areas - It is sometimes difficult to justify the use of electric actuators in a hazardous environment, but if compressed air is not available or if a pneumatic actuator will not provide the operating characteristics required, then an electric actuator with a properly classified enclosure may be used. 4. Speed - Pneumatic have fast actuation times, in electrical they need speed modulators 5. Cycle-life - Pneumatic have 1 million cycles, Electrical shorter cycle, 1000 to 15000 cycles Types of Actuators 1. Hydraulic Actuators - Hydraulic Actuators, as used in industrial process control, employ hydraulic pressure to drive an output member. - These are used where high speed and large forces are required. - The fluid used in hydraulic actuator is highly incompressible so that pressure applied can be transmitted instantaneously to the member EEE MITS attached to it. 26 Principle Used in Hydraulic Actuator System: Pascal’s Law - Pressure applied to a confined fluid at any point is transmitted undiminished and equally throughout the fluid in all directions and acts upon every part of the confining vessel at right angles to its interior surfaces. - Since pressure ‘P’ applied on an area ‘A’ gives rise to a force ‘F’, given as, F = P×A - Thus, if a force is applied over a small area to cause a pressure P in a confined fluid, the force generated on a larger area can be made many times larger than the applied force that created the pressure. - This principle is used in various hydraulic devices to such hydraulic press to generate very high forces. - A hydraulic actuator consists of a cylinder or fluid motor that uses hydraulic power to facilitate mechanical operation. - The mechanical motion gives an output in terms of linear, rotary or oscillatory motion. - Because liquids are nearly impossible to compress, a hydraulic actuator can exert considerable force. - The hydraulic cylinder consists of a hollow cylindrical tube along which a piston can slide. 27 - The term single acting is used when the fluid pressure is applied to just one side of the piston. - The piston can move in only one direction, a spring being frequently used to give the piston a return stroke. - The term double acting is used when pressure is applied on each side of the piston; any difference in pressure between the two sides of the piston moves the piston to one side or the other. 28 Components of Hydraulic Actuation Systems a. Hydraulic Fluid - Hydraulic fluid must be essentially non-compressible to be able to transmit power instantaneously from one part of the system to another. - At the same time, it should lubricate the moving parts to reduce friction loss and cool the components so that the heat generated does not lead to fire hazards. - It also helps in removing the contaminants to filter. - The most common liquid used in hydraulic systems is petroleum oil because it is only very slightly compressible. - The other desirable property of oil is its lubricating ability. - Finally, often, the fluid also acts as a seal against leakage inside a hydraulic component. 29 b. The Fluid Delivery Subsystem - It consists of the components that hold and carry the fluid from the pump to the actuator. - It is made up of the following components. Reservoir - It holds the hydraulic fluid to be circulated and allows air entrapped in the fluid to escape. - Oil deteriorates considerably in the presence of entrapped air bubbles. Filter - The hydraulic fluid is kept clean in the system with the help of filters and strainers. - It removes minute particles from the fluid, which can cause blocking of the orifices of servo-valves Line - Pipe, tubes and hoses, along with the fittings or connectors, constitute the conducting lines that carry hydraulic fluid between components. - Lines have higher power to weight ratio. 30 - Lines convey the fluid and also dissipate heat. - The working lines carry the fluid that delivers the main pump power to the load. - The pilot lines carry fluid that transmit controlling pressures to various directional and relief valves for remote operation or safety. - Lastly there are drain lines that carry the fluid that inevitably leaks out, to the tank. Fittings and Seals - To join pipe or tube sections, create bends - Also to prevent internal and external leakage in hydraulic systems. - External leakage, causes loss of fluid and can create fire hazards, as well as fluid contamination. Hydraulic Pumps - The pump converts the mechanical energy of its prime-mover to hydraulic energy by delivering a given quantity of hydraulic fluid at high pressure into the system. 31 Advantages • Hydraulic actuators are rugged and suited for high-force applications. They can produce forces 25 times greater than pneumatic cylinders of equal size. They also operate in pressures of up to 4,000 psi. • Hydraulic motors have high horsepower-to-weight ratio by 1 to 2 hp/lb greater than a pneumatic motor. • A hydraulic actuator can hold force and torque constant without the pump supplying more fluid or pressure due to the incompressibility of fluids • Hydraulic actuators can have their pumps and motors located a considerable distance away with minimal loss of power. Disadvantages • Hydraulics will leak fluid. Like pneumatic actuators, loss of fluid leads to less efficiency. However, hydraulic fluid leaks lead to cleanliness problems and potential damage to surrounding components and areas. • Hydraulic actuators require many companion parts, including a fluid reservoir, motors, pumps, release valves, and heat exchangers, along with noise-reduction equipment. This makes for linear motions systems that are large and difficult to accommodate. 32 2. Pneumatic actuators - Pneumatic actuators are devices used for converting pressure energy of compressed air into the mechanical energy to perform useful work. Principle of operation - A Pneumatic actuator mainly consists of a piston or a diaphragm which develops the motive power. - It keeps the air in the upper portion of the cylinder, allowing air pressure to force the diaphragm or piston to move the valve stem or rotate the valve control element. - Valves require little pressure to operate and usually double or triple the input force. - The larger the size of the piston, the larger the output pressure can be. - Having a larger piston can also be good if air supply is low, allowing the same forces with less input. - These pressures are large enough to crush objects in the pipe. - On 100 kPa input, you could lift a small car (upwards of 1,000 lbs) easily, and this is only a basic, small pneumatic valve. - However, the resulting forces required of the stem would be too great and cause the valve stem to fail. 33 - This pressure is transferred to the valve stem, which is connected to either the valve plug (see plug valve), butterfly valve etc. - Larger forces are required in high pressure or high flow pipelines to allow the valve to overcome these forces, and allow it to move the valves moving parts to control the material flowing inside. - The valves input is the "control signal." This can come from a variety of measuring devices, and each different pressure is a different set point for a valve. - A typical standard signal is 20–100 kPa. For example, a valve could be controlling the pressure in a vessel which has a constant out-flow, and a varied in-flow (varied by the actuator and valve). - A pressure transmitter will monitor the pressure in the vessel and transmit a signal from 20–100 kPa. - 20 kPa means there is no pressure, 100 kPa means there is full range pressure (can be varied by the transmitters calibration points). - As the pressure rises in the vessel, the output of the transmitter rises, this increase in pressure is sent to the valve, which causes the valve to stroke downward, and start closing the valve, decreasing flow into the vessel, reducing the pressure in the vessel as excess pressure is evacuated through the out flow. - This is called a direct acting process. 34 Components of Pneumatic Actuator Compressor – It act as power sources for pneumatic system. Air treatment unit – It consist of FRL unit which comprises of filter to filter the compressed air, a pressure regulator to regulate a flow of compressed air and a lubricator to lubricate air. Valve – It controls the direction and regulates the air flowing to actuating system Actuator – It convert the pneumatic force or compressed air pressure into mechanical force or movement. Pipes – Used to connect various elements of actuating system with each other. 35 Sensors – It act as feedback element for a controller in pneumatic system. - Pneumatic actuator consists of a piston and rod moving inside a closed cylinder. - This actuator style can be sub-divided into two types based on the operating principle: single acting and double acting. Single-acting cylinders • One working port. • Forward motion of the piston is obtained by supplying compressed air to working port. • Return motion of piston is obtained by spring placed on the rod side of the cylinder. • Are used where force is required to be exerted only in one direction. • Such as clamping, feeding, sorting, locking, ejecting, braking etc. • Available in short stroke lengths [maximum length up to 80 mm] due to the natural length of the spring. • Single Acting Cylinder exert force only in one direction. • Single acting cylinders require only about half the air volume consumed by a double acting cylinder for one operating cycle. 36 Double-acting cylinders - Equipped with two working ports- one on the piston side and the other on the rod side. - To achieve forward motion of the cylinder, compressed air is admitted on the piston side and the rod side is connected to exhaust. - During return motion supply air admitted at the rod side while the piston side volume is connected to the exhaust. - Force is exerted by the piston both during forward and return motion of cylinder. - Double acting cylinders are available in diameters from few mm to around 300 mm and stroke lengths of few mm up to 2 meters 37 Advantages Pneumatic actuators generate precise linear motion by providing accuracy, for example, within 0.1 inches and repeatability within .001 inches. Pneumatic actuators typical applications involve areas of extreme temperatures. A typical temperature range is -40°F to 250°F. In terms of safety and inspection, by using air, pneumatic actuators avoid using hazardous materials. They meet explosion protection and machine safety requirements because they create no magnetic interference due to their lack of motors. The cost of pneumatic actuators is low compared to other actuators. Disadvantages Pressure losses and air’s compressibility make pneumatics less efficient than other linear-motion methods. Compressor and air delivery limitations mean that operations at lower pressures will have lower forces and slower speeds. A compressor must run continually operating pressure even if nothing is moving. To be truly efficient, pneumatic actuators must be sized for a specific job. Hence, they cannot be used for other applications. Accurate control and efficiency requires proportional regulators and valves, but this raises the costs and complexity. Even though the air is easily available, it can be contaminated by oil or lubrication, leading to downtime and maintenance. Companies still have to pay for compressed air, making it a 38 consumable, and the compressor and lines are another maintenance issue. 3. Electrical Actuator - Electrical Actuator is an electromechanical device that converts electrical energy into mechanical energy. - Most electric actuators operate through the interaction of magnetic fields and current-carrying conductors to generate force. - The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo; some electric actuators can also be used as generators, for example, a traction motor on a vehicle may perform both tasks. Main Components of an Electrical Actuator - The motor of an electric actuator can operate on any voltage and is used across many different industries. The most common voltages used in single-phase motors are 12-24V DC and 24-230V AC. In addition to these options, three-phase motors also used. - An actuator’s brake is mounted on top of the motor. It’s responsible for stopping the media from forcing the valve open when it should be closed by locking the motor rotor in position when not in use. - The motor start capacitor is the third main component of an electric actuator with AC motor. It gives the motor enough power to start. Electric actuators with DC motors do not require a capacitor. 39 - The final critical component of an electric actuator is the two single-pole, double-throw (SPDT) end of travel limit switches. - One is for the closed position, and the other is for the open position. - These switches provide the adequate number of electrical contacts in the immediate, closed, and open positions. i) Linear electric actuators - These provide linear motion via a motor- driven ball screw or screw assembly. - The linear actuator’s load is attached to the end of a screw, or rod, and is unsupported. - The screw can be direct, belt, or gear driven. - Motor choices include DC, DC servo, DC brushless, DC brushless servo, AC, AC servo, and stepper. - Input power can be specified for DC, AC, or stepper motors. - Drive screw specifications to consider for linear actuators include drive screw type and screw lead. 40 - Features include self-locking, limit switches, motor encoder feedback and linear position feedback. - Screw choices include acme screws and ball screws. - Acme screws will typically hold loads without power but are usually less efficient than ball screws. - They also typically have a shorter life but are more robust to shock loads. - Other features for linear actuators to consider include holding brakes, integrated overload slip clutch or torque limiters, water-resistant construction, protective boot and thermal overload protection. ii) Rotary electric actuators - These provide incremental rotational movement of the output shaft. - In its most simple form, a rotary actuator consists of a motor with a speed reducer. - These AC and DC motors can be fabricated to the exact voltage, frequency, power, and performance specified. - The speed reducer is matched with the ratio to the speed, torque, and acceleration required. - Life, duty cycle, limit load, and accuracy are considerations that further define the selection of the speed reducer. 41 - Hardened, precision spur gears are supported by antifriction bearings as a standard practice in these speed reducers. - Compound gear reduction is accomplished in compact, multiple load path configurations, as well as in planetary forms. - The specifications for rotary actuator include angular rotation, torque, and speed, as well as control signals and feedback signals, and the environment temperature. - Rotary actuators can incorporate a variety of auxiliary components such as brakes, clutches, ant backlash gears, and/or special seals. Advantages • Electrical actuators offer the highest precision-control positioning. An example of the range of accuracy is +/- 0.000315 in. and a repeatability of less than 0.0000394 in. Their setups are scalable for any purpose or force requirement, and are quiet, smooth and repeatable. • Electric actuators can be networked and reprogrammed quickly. They offer immediate feedback for diagnostics and maintenance. • They provide complete control of motion profiles and can include encoders to control velocity, position, torque, and applied force. • In terms of noise, they are quieter than pneumatic and hydraulic actuators EEE MITS 42 • Because there are no fluids leaks, environmental hazards are eliminated. Disadvantages • The initial unit cost of an electrical actuator is higher than that of pneumatic and hydraulic actuators. • Electrical actuators are not suited for all environments, unlike pneumatic actuators, which are safe in hazardous and flammable areas • A continuously running motor will overheat, increasing wear and tear on the reduction gear. The motor can also be large and create installation problems. • The motor chosen locks in the actuator’s force, thrust, and speed limits to a fixed setting. If a different set of values for force, thrust, and speed are desired, the motor must be changed. 4. Electro-Pneumatic Actuator - Electro pneumatics is now commonly used in many areas of Industrial low cost automation. - They are also used extensively in production, assembly, pharmaceutical, chemical and packaging systems. - There is a significant change in controls systems. - Relays have increasingly been replaced by the programmable logic controllers in order to meet the growing demand for more flexible automation. - Electro-pneumatic control consists of electrical control systems operating pneumatic power 43 systems. - In this solenoid valves are used as interface between the electrical and pneumatic systems. - Devices like limit switches and proximity sensors are used as feedback elements. - Electro Pneumatic control integrates pneumatic and electrical technologies, is more widely used for large applications. - In Electro Pneumatics, the signal medium is the electrical signal either AC or DC source is used. - Working medium is compressed air. - Operating voltages from around 12 V to 220 Volts are often used. - The final control valve is activated by solenoid actuation. - The resetting of the valve is either by spring [single Solenoid] or using another solenoid [Double solenoid Valve]. - More often the valve actuation/reset is achieved by pilot assisted solenoid actuation to reduce the size and cost of the valve Control of Electro Pneumatic system is carried out either using combination of Relays and Contactors or with the help of Programmable Logic Controllers [PLC]. - A Relay is often is used to convert signal input from sensors and switches to number of output signals [either normally closed or normally open] - Signal processing can be easily achieved using relay and contactor combinations 44 - A Programmable Logic Controller can be conveniently used to obtain the outputs as per the required logic, time delay and sequential operation. - Finally the output signals are supplied to the solenoids activating the final control valves which controls the movement of various cylinders. - The greatest advantage of electro pneumatics is the integration of various types of proximity sensors [electrical] and PLC for very effective control. - As the signal speed with electrical signal, can be much higher, cycle time can be reduced and signal can be conveyed over long distances. - In Electro pneumatic controls, mainly three important steps are involved: Signal input devices - Signal generation such as switches and contactor, various types of contact and proximity sensors Signal Processing – Use of combination of Contactors of Relay or using Programmable Logic Controllers Signal Outputs – Outputs obtained after processing are used for activation of solenoids, indicators or audible alarms 45 Solenoids - Electrically actuated directional control valves form the interface between the two parts of an electro pneumatic control. - The most important tasks of electrically actuated DCVs include. i) Switching supply air on or off ii) Extension and retraction of cylinder drives - Electrically actuated directional control valves are switched with the aid of solenoids. They can be divided into two groups: i) Spring return valves only remain in the actuated position as long as current flows through the solenoid ii) Double solenoid valves retain the last switched position even when no current flows through the solenoid. - In the initial position, all solenoids of an electrically actuated DCVs are de-energized and the solenoids are inactive. - A double valve has no clear initial position, as it does not have a return spring. - The possible voltage levels for solenoids are 12 V DC, 12V AC, 12 V 50/60 Hz, 24V 50/60 Hz, 110/120V 50/60 Hz, 220/230V 50/60 Hz. 46 Valves - A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. - Valves are technically fittings. - In an open valve, fluid flows in a direction from higher pressure to lower pressure. - A control valve is normally driven by a diaphragm type pneumatic actuator that throttles the flow of the manipulating variable for obtaining the desired control action. - A control valve essentially consists of a plug and a stem. - The stem can be raised or lowered by air pressure and the plug changes the effective area of an orifice in the flow path. - A typical control valve action can be explained using given figure. - When the air pressure increases, the downward force of the diaphragm moves the stem downward against the spring. 47 - Control valves are available in different types and shapes based on: (a) Action (b) Number of plugs (c) Flow characteristics. (a). Based on Action - Control valves operated through pneumatic actuators can be either (i) Air to open or (ii) Air to close. - They are designed such that if the air supply fails, the control valve will be either fully open, or fully closed, depending upon the safety requirement of the process. - For example, if the valve is used to control steam or fuel flow, the valve should be shut off completely in case of air failure. - On the other hand, if the valve is handling cooling water to a reactor, the flow should be maximum in case of emergency. - The schematic arrangements of these two actions are shown in figure 48 - Valve A are air to close type, indicating, if the air fails, the Valve will be fully open. - Opposite is the case for valve B. (b) Based on Number of plugs i. single-seated valve - only one plug is present in the control valve - Advantage of this type of valve is that, it can be fully closed and flow variation from 0 to 100% can be achieved. - But looking at its construction, due to the pressure drop across the orifice a large upward force is present in the orifice area, and as a result, the force required to move the valve against this upward thrust is also large. - Suitable for small flow rates. ii. Double-seated valve. - There are two plugs; flow moves upward in one orifice area, and downward in the other orifice. The resultant upward or downward thrust is almost zero. - As a result, the force required to move a double-seated valve is comparatively much less. - The flow cannot be shut off completely, because of the differential temperature expansion of the stem and the valve seat. 49 - If one plug is tightly closed, there is usually a small gap between the other plug and its seat. - Thus, single-seated valves are recommended for when the valves are required to be shut off completely. 50 Valve actuator - A valve actuator is the mechanism for opening and closing a valve. i) Manually operated valves - Require someone to adjust them using a direct or geared mechanism attached to the valve stem. ii) Power-operated valves - Using gas pressure, hydraulic pressure or electricity - Allow a valve to be adjusted remotely, or allow rapid operation of large valves. - The final elements of an automatic control loop - Automatically regulates flow, level or other process. - Actuators may be only to open and close the valve, or may allow intermediate positioning - Some valve actuators include switches or other ways to remotely indicate the position of the valve. 51 Valve selection criteria - The control valves are selected based on the following criterias • Type of fluid to be controlled. • Temperature range of fluid. • Viscosity range of fluid. • Specific gravity range of fluid. • Minimum and maximum flow required. • Minimum and maximum inlet pressure at the control valve. • Minimum and maximum outlet pressure at the control valve. • Pressure drop across the valve expected during normal flowing conditions. • Pressure drop across the valve at zero flow. • Maximum permissible noise level, if pertinent, and the measurement reference point. • Degrees of superheat or existence of flashing across the valve, if known. • Inlet and outlet pipeline size and schedule of pipe 52 Solenoid actuator - Solenoid Actuator also called as ON/OFF valve or Solenoid Valve. - A typical electric solenoid actuator is shown in Figure. - It consists of a coil, armature, spring, and stem. - The coil is connected to an external current supply. - The spring rests on the armature to force it downward. - The armature moves vertically inside the coil and transmits its motion through the stem to the valve. - When current flows through the coil, a magnetic field forms around the coil. - The magnetic field attracts the armature toward the center of the coil. - As the armature moves upward, the spring collapses and the valve opens. - When the circuit is opened and current stops flowing to the coil, the magnetic field collapses. - This allows the spring to expand and shut the valve. 53 - A major advantage of solenoid actuators is their quick operation. - Also, they are much easier to install than pneumatic or hydraulic actuators. - However, solenoid actuators have two disadvantages. - First, they have only two positions: fully open and fully closed. - Second, they don’t produce much force, so they usually only operate relatively small valves. 54 Converting an open loop system to an automatic system - A control system in which the control action is totally independent of the output of the system then it is called an open-loop control system. - A manual control system is also an open-loop control system. Closed Loop Control/Automated System - Control systems in which the output has an effect on the input quantity in such a manner that the input quantity will adjust itself based on the output generated is called a closed-loop control system. - An open-loop control system can be converted into a closed loop control system by providing feedback. - This feedback automatically makes suitable changes in the output due to external disturbance. - In this way, a closed loop control system is called an automatic control system. 55 - The figure below shows the block diagram of the closed loop control system in which feedback is taken from the output and fed into the input. Example : Electrical Furnace Open Loop System: - The electric furnace shown in figure is an open loop system. - The output in the system is the desired temperature. - The temperature of the system is raised by heat generated by the heating element. 56 - The output temperature depends on the time during which the supply to heater remains ON. - The ON and OFF of the supply is governed by the time setting of the relay. Closed Loop System: - The electric furnace shown in the below figure is a closed loop system. - The output of the closed loop system is the desired temperature and it depends on the time during which the supply to heater remains ON. 57 - The switching ON and OFF of the relay is controlled by a controller which is a digital system or computer. - The desired temperature is input to the system through the keyboard or as a signal corresponding to the desired temperature via ports. - The actual temperature is sensed by Sensor and converted to digital signal by the A/D converter. - The computer reads the actual temperature and compares with the desired temperature. - If it finds any difference then it sends the signal to switch ON or OFF the relay through D/A converter and amplifier. - Thus the system automatically corrects any changes in output. - Hence it is a closed loop system. 58 Shape Memory Alloys (SMA) - Shape memory alloys are a unique class of metal alloys that can recover apparent permanent strains when they are heated above a certain temperature. - A shape memory alloy (SMA, smart metal, memory metal, memory alloy, muscle wire, smart alloy) is an alloy that "remembers" its original, cold-forged shape: returning the predeformed shape by heating. - This material is a lightweight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, and motor-based systems. - Shape memory alloys have applications in industries including medical and aerospace. - The shape memory alloys have two stable phases - the high–temperature phase, called Austenite and the low–temperature phase, called Martensite. 59 - The three main types of shape memory alloys are the copper-zinc aluminium-nickel, copper-aluminium-nickel, and nickel-titanium (NiTi) alloys but SMA's can also be created by alloying zinc, copper, gold, and iron. - NiTi alloys are generally more expensive and change from austenite to Martensite upon cooling; Mf is the temperature at which the transition to Martensite is finished during cooling. - Accordingly, during heating As and Af are the temperatures at which the transformation from Martensite to Austenite starts and finishes. - Repeated use of the shape memory effect may lead to a shift of the characteristic transformation temperatures (this effect is known as functional fatigue, as it is closely related with a change of microstructural and functional properties of the material) 60 - In this figure, ξ (T) represents the Martensite fraction. - The difference between the heating transition and the cooling transition gives rise to the hysteresis effect where some of the mechanical energy is lost in the process. - The shape of the curve depends on the material properties of the shape memory alloy, such as the alloying and work hardening. Types of SMA’s i) One way SMA ii) Two way SMA i) One way memory effect - When a shape memory alloy is in its cold state (below As), the metal can be bent or stretched and will hold this shape until heated above the transition temperature. - Upon heating, the shape changes to its original. - When the metal cools again, it will remain in the hot shape until deformed again. - In this case, cooling from high temperature does not cause macroscopic shape change. 61 ii) Two way memory effect - This is the effect that the material remembers two shapes: one at high temp and the other at low temperature. - These metals show shape memory effect during both cooling and heating. - The metal can be trained to leave some reminders of the deformed low temp condition in the high temp phases. - Above a certain temp, the metal loses the 2 way memory effect. This is called “amnesia”. 62 Pseudo‐Elasticity - One of the commercial uses of shape memory alloy involves using the pseudo-elastic properties of the metal during the high temperature (austenitic) phase. - The frames of reading glasses have been made of shape memory alloy as they can undergo large deformations in their high temperature state and then instantly revert back to their original shape when the stress is removed. - This is the result of pseudo elasticity; the martensitic phase is generated by stressing the metal in the austenitic state and this martensite phase is capable of large strains. - With the removal of the load, the martensite transforms back into the austenite phase and resumes its original shape. - This allows the metal to be bent, twisted and pulled, before reforming its shape when released. - This means the frames of shape memory alloy glasses are claimed to be "nearly indestructible" because it appears no amount of bending results in permanent plastic deformation. Manufacture - Shape memory alloys are typically made by casting, using vacuum arc melting or induction melting. - These are specialist techniques used to keep impurities in the alloy to a minimum and ensure the metals are well mixed. 63 - The ingot is then hot rolled into longer sections and then drawn to turn it into wire. - The way in which the alloys are "trained" depends on the properties wanted. - The "training" dictates the shape that the alloy will remember when it is heated. - This occurs by heating the alloy so that the dislocations re-order into stable positions. - They are then shaped while hot and are cooled rapidly by quenching in water or by cooling with air. Applications • Aircraft Boeing, General Electric Aircraft Engines, Goodrich Corporation, NASA, and All Nippon Airways developed the Variable Geometry Chevron using shape memory alloy that reduces aircraft's engine noise. • Robotics There have also been limited studies on using these materials in robotics (such as "Roboterfrau Lara"), as they make it possible to create very light robots. Weak points of the technology are energy inefficiency, slow response times, and large hysteresis. • Medicine Shape memory alloys are applied in medicine, for example, as fixation devices for osteotomies in orthopedic surgery, in dental braces to exert constant tooth-moving forces on the teeth and in stent grafts where it gives the ability to adapt to the shape of certain EEE MITS 64 blood vessels when exposed to body temperature. Previous University Questions 1. Explain the selection criterion for control valves? 2. 3. 4. Define an industrial automation system and enlist its components. What is the role of actuators in automation system? How they are classified? How can you convert an open loop system to an automatic system? Explain with the help of an example. 5. 6. 7. With the help of a neat diagram explain the working of butterfly valve. Explain the characteristic features of shape memory alloy. Give the classification of control valves. 8. 9. Explain the working of a solenoid actuator with the help of diagram. With the help of a block diagram explain the working of an automated system. 65